This invention pertains to refractory clad metallurgical lances particularly useful in the processing of molten metal such as steel. Refractory clad metallurgical lances are shown in U.S. Pat. Nos. 3,292,622, 3,833,209, and 3,976,286.
Steel is produced in a refining vessel such as an electric or basic oxygen furnace. When the proper temperature and chemistries of the molten metal are achieved, the superheated steel is tapped from the furnace into a transfer ladle for the purpose of degassing the molten metal and performing related processing functions commonly known as temperature trimming to cool and stir the melt and to simultaneously insure a homogenous composition. In such stirring operations, a metallurgical lance is adapted to transmit inert gas such as nitrogen or argon under pressure to a nozzle portion of a metallurgical lance partially immersed in the molten metal. Argon gas is preferred to avoid nitrogen pickup by certain kinds of steel grades. In use, the lance is inserted within the ladle until the lowermost portion of the lance containing a discharge opening is disposed below the molten steel whereby agitation of the molten metal becomes vigorous due to pressurized gas passing through the lance and bubbling through the molten metal. The temperature of the molten metal is taken periodically during the stirring process, by an expandable thermocouple which is then discarded after each use. After various trimmings have been performed, additional temperatures must be taken. Temperature control is necessary to obtain the intended chemical and metallurgical results as well as consistently high quality steel. Depending on how and where the steel is treated following the ladle transfer, the molten metal must be maintained above certain critical temperature or minimum temperature to avoid metallurgical problems which can occur if the melt is too cold during the stirring process step. Expendable batch thermometers or thermocouples, however, are costly and provide only instantaneous temperature readings and often provide misleading temperature measurements due to cooling. Temperature control is unsatisfactory due to intermittent spot readings. The expendable thermometers are destroyed in use thus requiring several thermometers to be used in the manufacture of a single batch of steel. U.S. Pat. No. 3,413,852 suggests an oxygen lance containing a plurality of radiometers adapted to indirectly measure molten metal surface temperatures and radiation gas passing through the oxygen lance. Such radiometers, however, are expensive devices and merely provide indirect measurement of molten metal temperatures causing inaccuracies associated with radiation measurements such as emissivity corrections, convection losses, heat transfer errors, molten metal surface phenomena and related discrepancies inherent with indirect radiation measurements of molten steel by optical measuring methods.
Accordingly, it is highly desirable to provide a means for directly measuring temperature of molten metal accurately and for continuously monitoring the direct melt temperature during the stirring process, and to further provide such means for direct and continuous temperature measurements in combination with the matallurgical lance. The refractory cladding effectively withstands the extremely high heat, turbulence of the molten metal, thermal stresses due to rapid heating and cooling as well as chemical erosion, which can cause premature failure of the metallurgical lance. Even more advantageous, it now has been found that a thermocouple imbedded within the refractory material of a refractory clad metallurgical lance can provide accurate and continuous direct temperature measurements of the molten metal simultaneously with the operation of the lance, and additionally overcomes the problem of using numerous expendable thermocouples.